In a CDMA communication system, transmit power control is a technology essential to lessening a so-called near-far problem and increasing a subscriber capacity.
For example, with regard to a downlink, transmit power of a base station needs to be set to a minimum level necessary to guarantee communication quality to reduce interference between mobile stations. The level of interference between mobile stations determines the subscriber capacity of the base station.
The subscriber capacity in a CDMA communication system is determined by modeling a cell/sector configuration and propagation loss, etc., and estimating the probability that transmit power which is determined according to the state of a mobile station will exceed maximum transmit power of the base station.
Such calculations are complicated and always involve ambiguity. A W-CDMA system needs to secure communication quality and secure a maximum number of subscribers as well. Therefore, insufficient accuracy of transmit power control may cause a failure of the system.
When closed loop control of transmit power is applied, both the base station and mobile station perform transmit power control based on a TPC bit (transmit power control bit) included in a received signal.
As described above, transmit power control using a TPC bit is required to have the highest possible degree of accuracy.
However, the TPC bit itself actually includes errors for various reasons. The inventor of the present application has noticed a conversion error of an AD converter as one of the causes for such errors.
That is, a received signal is affected by fading and its amplitude often fluctuates a great deal.
At this time, if the amplitude of a received signal exceeds the dynamic range of an AD converter at the input stage of a receiver and at the same time amplitude adjustment by AGC (auto gain control) cannot follow its amplitude variations, the conversion output of the AD converter is clamped by a maximum value or minimum value of the output of the AD converter, causing a small amount of loss of information.
Reception processing and generation of transmit power control information based on the data containing such errors caused by AD conversion give rise to errors in transmit power control.
No matter how small those errors may be, when errors included in TPC bits sent from many mobile stations are accumulated, there is a danger that the accuracy of transmit power control at the base station may decrease, making it impossible to secure the number of subscribers acceptable to the system.
For example, gain correction by an amplifier before the receiver through feedback control (AGC) cannot absorb quicker variations in the received signal than a time constant of control. Therefore, when intensity of the reception field in an arbitrary section is measured, variations cannot be absorbed by gain correction of the amplifier and the amplitude level of the received signal exceeds the dynamic range of the AD converter, failing to obtain an accurate AD conversion result and resulting in errors included in the calculated field intensity value itself.
The reception field intensity not only serves as means to know the current reception level at the mobile terminal but also forms a basis for generation of transmission control information to be sent to the base station and has an extremely important meaning. Therefore, the presence of errors in the reception field intensity may become a cause for a reduction of reception quality. This is especially noticeable in a CDMA scheme which controls power of transmission and reception meticulously.
That is, the number of users of the CDMA communication system is increasing drastically in recent years, and pursuing communication quality and the number of subscribers to the maximum with consideration given to such an increase of subscribers leads to a problem that a tiny conversion error (latent error) due to saturation of an AD converter, which has been overlooked conventionally, may also cause errors in the mobile communication system itself.